Add trivial cast lints.

This permits all coercions to be performed in casts, but adds lints to warn in those cases.

Part of this patch moves cast checking to a later stage of type checking. We acquire obligations to check casts as part of type checking where we previously checked them. Once we have type checked a function or module, then we check any cast obligations which have been acquired. That means we have more type information available to check casts (this was crucial to making coercions work properly in place of some casts), but it means that casts cannot feed input into type inference.

[breaking change]

* Adds two new lints for trivial casts and trivial numeric casts, these are warn by default, but can cause errors if you build with warnings as errors. Previously, trivial numeric casts and casts to trait objects were allowed.
* The unused casts lint has gone.
* Interactions between casting and type inference have changed in subtle ways. Two ways this might manifest are:
- You may need to 'direct' casts more with extra type information, for example, in some cases where `foo as _ as T` succeeded, you may now need to specify the type for `_`
- Casts do not influence inference of integer types. E.g., the following used to type check:

```
let x = 42;
let y = &x as *const u32;
```

Because the cast would inform inference that `x` must have type `u32`. This no longer applies and the compiler will fallback to `i32` for `x` and thus there will be a type error in the cast. The solution is to add more type information:

```
let x: u32 = 42;
let y = &x as *const u32;
```

7 files changed, 12 insertions, 11 deletions

diff --git a/src/libsyntax/codemap.rs b/src/libsyntax/codemap.rs
index 1abe8d0a3c1..6a00fff1860 100644
--- a/src/libsyntax/codemap.rs
+++ b/src/libsyntax/codemap.rs
@@ -284,7 +284,7 @@ pub const COMMAND_LINE_EXPN: ExpnId = ExpnId(-2);
 
 impl ExpnId {
     pub fn from_llvm_cookie(cookie: c_uint) -> ExpnId {
-        ExpnId(cookie as u32)
+        ExpnId(cookie)
     }
 
     pub fn to_llvm_cookie(self) -> i32 {
@@ -376,7 +376,7 @@ impl Encodable for FileMap {
                         match bytes_per_diff {
                             1 => for diff in diff_iter { try! { (diff.0 as u8).encode(s) } },
                             2 => for diff in diff_iter { try! { (diff.0 as u16).encode(s) } },
-                            4 => for diff in diff_iter { try! { (diff.0 as u32).encode(s) } },
+                            4 => for diff in diff_iter { try! { diff.0.encode(s) } },
                             _ => unreachable!()
                         }
                     }
@@ -650,7 +650,7 @@ impl CodeMap {
         let lo = self.lookup_char_pos(sp.lo);
         let hi = self.lookup_char_pos(sp.hi);
         let mut lines = Vec::new();
-        for i in lo.line - 1..hi.line as usize {
+        for i in lo.line - 1..hi.line {
             lines.push(i);
         };
         FileLines {file: lo.file, lines: lines}
diff --git a/src/libsyntax/ext/base.rs b/src/libsyntax/ext/base.rs
index 35449bde0b2..b679456b353 100644
--- a/src/libsyntax/ext/base.rs
+++ b/src/libsyntax/ext/base.rs
@@ -264,7 +264,7 @@ macro_rules! make_MacEager {
                     box MacEager {
                         $fld: Some(v),
                         ..Default::default()
-                    } as Box<MacResult>
+                    }
                 }
             )*
         }
@@ -330,7 +330,7 @@ impl DummyResult {
     /// Use this as a return value after hitting any errors and
     /// calling `span_err`.
     pub fn any(sp: Span) -> Box<MacResult+'static> {
-        box DummyResult { expr_only: false, span: sp } as Box<MacResult+'static>
+        box DummyResult { expr_only: false, span: sp }
     }
 
     /// Create a default MacResult that can only be an expression.
@@ -339,7 +339,7 @@ impl DummyResult {
     /// if an error is encountered internally, the user will receive
     /// an error that they also used it in the wrong place.
     pub fn expr(sp: Span) -> Box<MacResult+'static> {
-        box DummyResult { expr_only: true, span: sp } as Box<MacResult+'static>
+        box DummyResult { expr_only: true, span: sp }
     }
 
     /// A plain dummy expression.
diff --git a/src/libsyntax/ext/quote.rs b/src/libsyntax/ext/quote.rs
index c11ffe66e6c..1aceec310e1 100644
--- a/src/libsyntax/ext/quote.rs
+++ b/src/libsyntax/ext/quote.rs
@@ -262,6 +262,7 @@ pub mod rt {
         (unsigned, $t:ty, $tag:expr) => (
             impl ToSource for $t {
                 fn to_source(&self) -> String {
+                    #![allow(trivial_numeric_cast)]
                     let lit = ast::LitInt(*self as u64, ast::UnsignedIntLit($tag));
                     pprust::lit_to_string(&dummy_spanned(lit))
                 }
diff --git a/src/libsyntax/ext/tt/macro_rules.rs b/src/libsyntax/ext/tt/macro_rules.rs
index 5940b791843..1e53db60301 100644
--- a/src/libsyntax/ext/tt/macro_rules.rs
+++ b/src/libsyntax/ext/tt/macro_rules.rs
@@ -169,7 +169,7 @@ fn generic_extension<'cx>(cx: &'cx ExtCtxt,
                 // Weird, but useful for X-macros.
                 return box ParserAnyMacro {
                     parser: RefCell::new(p),
-                } as Box<MacResult+'cx>
+                }
               }
               Failure(sp, ref msg) => if sp.lo >= best_fail_spot.lo {
                 best_fail_spot = sp;
diff --git a/src/libsyntax/parse/lexer/mod.rs b/src/libsyntax/parse/lexer/mod.rs
index e0953a8ace6..0843713681b 100644
--- a/src/libsyntax/parse/lexer/mod.rs
+++ b/src/libsyntax/parse/lexer/mod.rs
@@ -758,7 +758,7 @@ impl<'a> StringReader<'a> {
                 self.err_span_char(self.last_pos, self.pos,
                               "illegal character in numeric character escape", c);
                 0
-            }) as u32;
+            });
             self.bump();
         }
 
@@ -887,7 +887,7 @@ impl<'a> StringReader<'a> {
                     self.fatal_span_char(self.last_pos, self.pos,
                                    "illegal character in unicode escape", c);
                 }
-            }) as u32;
+            });
             self.bump();
             count += 1;
         }
diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index 4ae5e0faa31..44097ffeb20 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -865,7 +865,7 @@ impl<'a> Parser<'a> {
         } else {
             // Avoid token copies with `replace`.
             let buffer_start = self.buffer_start as usize;
-            let next_index = (buffer_start + 1) & 3 as usize;
+            let next_index = (buffer_start + 1) & 3;
             self.buffer_start = next_index as isize;
 
             let placeholder = TokenAndSpan {
diff --git a/src/libsyntax/print/pprust.rs b/src/libsyntax/print/pprust.rs
index 828d085fd43..5941f044a07 100644
--- a/src/libsyntax/print/pprust.rs
+++ b/src/libsyntax/print/pprust.rs
@@ -2831,7 +2831,7 @@ impl<'a> State<'a> {
             ast::LitBinary(ref v) => {
                 let mut escaped: String = String::new();
                 for &ch in &**v {
-                    escaped.extend(ascii::escape_default(ch as u8)
+                    escaped.extend(ascii::escape_default(ch)
                                          .map(|c| c as char));
                 }
                 word(&mut self.s, &format!("b\"{}\"", escaped))